Christopher K. H. Guay

Flow‐weighted values of runoff tracers (δ18O, DOC, Ba, alkalinity) from the six largest Arctic rivers

dissolved organic carbon (DOC), dissolved barium and total alkalinity from the six largest Arctic rivers: the Ob’, Yenisey, Lena, Kolyma, Yukon and Mackenzie. These data, which can be used to trace runoff, are based upon coordinated collections between 2003 and 2006 that were temporally distributed to capture linked seasonal dynamics of river flow and tracer values. Individual samples indicate significant variation in the contributions each river makes to the Arctic Ocean. Use of these new flow-weighted estimates should reduce uncertainties in the analysis of freshwater transport and fate in the upper Arctic Ocean, including the links to North Atlantic thermohaline circulation, as well as regional water mass analysis. Additional improvements should also be possible for assessing the mineralization rate of the globally significant flux of terrigenous DOC contributed to the Arctic Ocean by these major rivers. Citation: Cooper, L. W., J. W. McClelland, R. M. Holmes, P. A. Raymond, J. J. Gibson, C. K. Guay, and B. J. Peterson (2008), Flow-weighted values of runoff tracers (d 18 O, DOC, Ba, alkalinity) from the six largest Arctic rivers, Geophys. Res. Lett., 35, L18606, doi:10.1029/2008GL035007.

Barium as a tracer of Arctic halocline and river waters

Dissolved barium (Ba) concentrations are reported for water samples collected during six oceanographic cruises to the Arctic in 1993 and during the 1994 Arctic Ocean Section. Upper Arctic ( -75 nmol Ba 1−1) observed in the surface mixed layer of the Arctic interior (i.e. beyond the 200 m isobath) occurred in the Canada Basin. As a result of the biological removal of Ba from surface waters of the Chukchi Sea and the tendency of the Bering inflow to enter the Arctic interior at depths below the relatively fresher mixed layer, we hypothesize that the Mackenzie River is the dominant source of the high Ba observed in the surface waters of the Canada Basin. Features characterized by local salinity minima and Ba maxima in surface waters down to depths of 30–50 m were observed over the Arlis Plateau and Mendeleyev Ridge in both 1993 and 1994, and over the Makarov Basin and Lomonosov Ridge in 1994. The physical and chemical properties of these waters suggest that they have been significantly influenced by fluvial discharge. It could not be determined from available data whether these features arose from offshore transport of Eurasian river discharge from the Russian Arctic seas, meanders of the Mackenzie River-influenced Beaufort Gyre, or a branch of Bering inflow that crossed the Chukchi shelf and entrained river water from North American and/or Eurasian sources. In both 1993 and 1994 Ba concentrations in the upper halocline layer (as defined by a core salinity of 33·1) ranged between 42 and 77 nmol Ba 1−1, with highest values observed in the Canada Basin. The Ba maximum and the related nutrient maxima and oxygen minimum that characterize the upper halocline in the Canada Basin suggest sources in the Chukchi and Beaufort seas, and that the mechanism by which these waters are formed is linked with the influx of water through the Bering Strait. The Ba distributions for 1993 and 1994 delineate the lateral extent of upper halocline waters with Canada Basin character and show them to be chemically distinct from upper halocline waters in the Eurasian Basin. The front separating these water types ran roughly along the Mendeleyev and Alpha ridges, consistent with reports that the boundary between Atlantic and Pacific water mass assemblies in the Arctic has recently been displaced from its historical position over the Lomonosov Ridge. Lateral gradients in both Ba and the NO parameter were observed within the lower halocline layer (defined by an NO minimum and core salinity of 34·2–34·4) along the 1994 Arctic Ocean Section, indicating that lower halocline waters receive additional inputs as they transit to the Canadian Basin from their region of formation in the Eurasian sector of the Arctic.

Wind-driven transport pathways for Eurasian Arctic river discharge

Distributions of temperature, salinity, and barium in near-surface waters (depth ≤ 50 m) of the Laptev Sea and adjacent areas of the Arctic Ocean are presented for the summers of 1993, 1995, and 1996. The tracer data indicate that while fluvial discharge was largely confined to the shelf region of the Laptev Sea in the summer of 1993, surface waters containing a significant fluvial component extended beyond the shelf break and over the slope and basin areas north of the Laptev Sea in the summers of 1995 and 1996. These distributions of fluvial discharge are consistent with local winds and suggest two principal pathways by which river waters can enter the central Arctic basins from the Laptev Sea. When southerly to southeasterly wind conditions prevail, river waters are transported northward beyond the shelf break and over the slope and adjacent basin areas. These waters can then enter the interior Arctic Ocean via upper layer flow in the vicinity of the Lomonosov Ridge. Under other wind conditions, river waters are steered primarily along the inner Laptev shelf and into the East Siberian Sea as part of the predominantly eastward coastal current system. These waters then appear to cross the shelf and enter the interior Arctic Ocean via upper layer flow aligned roughly along the Mendeleyev Ridge. The extent to which either pathway is favored in a given year is largely determined by local wind patterns during the summer months, when fluvial discharge is greatest and shelf waters are at the lowest salinity of their annual cycle.

A survey of dissolved barium in the estuaries of major Arctic rivers and adjacent seas

Analyses for dissolved Ba have been performed on water samples obtained from the estuaries of major Arctic rivers and adjacent seas between 1993 and 1996. Complex behavior was observed in the mixing zones between fluvial and oceanic waters. In these areas, riverine dissolved Ba signals were altered by mixing with different shelf water masses and by non-conservative processes such as uptake in association with biological activity and desorption from suspended riverine particles. Despite this complexity, dissolved Ba concentrations [Bad] measured in samples associated with the Mackenzie River (138–574 nmol L-1) were clearly much higher than those measured in samples associated with any of the Eurasian Arctic rivers (12–175 nmol L-1). While the [Bad] signatures of Arctic rivers are subject to temporal and seasonal variability, such variability appears to be much smaller than the overall difference between [Bad] in the Mackenzie River and Eurasian Arctic rivers. These results suggest that Ba can be used to distinguish between North American and Eurasian components of fluvial discharge to the Arctic and thus provide information not available from other oceanographic tracers currently in use.

High‐resolution measurements of dissolved organic carbon in the Arctic Ocean by in situ fiber‐optic spectrometry

Here we report results from an extensive survey of dissolved organic carbon (DOC) in the Arctic Ocean, which was achieved by means of a high-resolution, in situ UV fluorometer deployed on a nuclear submarine. Based on a strong linear correlation observed between fluorescence (320 nm excitation, 420 nm emission) and organic carbon concentrations determined directly by high-temperature combustion, a continuous record of DOC was produced at a keel depth of 58 m along a 2900-km transect north of the Beaufort, Chukchi, East Siberian and Laptev seas. The DOC record, combined with other physical and chemical measurements, identifies areas where river waters cross the shelves and enter the circulation of the Arctic interior. Fluvial sources were found to account for 12-56% of the total DOC in parts of the upper Makarov and Amundsen basins.

Measurements of Cd, Cu, Pb and Zn in the lower reaches of major Eurasian arctic rivers using trace metal clean techniques

Concentrations of dissolved and particulate Cd, Cu, Pb and Zn were determined in samples collected in summer 1998 from the lower reaches of six major Eurasian arctic rivers: the Onega, Severnaya Dvina, Mezen, Pechora, Ob and Yenisey. These data comprise some of the earliest measurements of trace metals in Eurasian arctic rivers above the estuaries using recognized clean techniques. Significant (alpha = 0.05) differences were observed among mean concentrations of particulate metals in the individual rivers (F < or = 0.006), with highest levels overall observed in the Severnaya Dvina and Yenisey. No significant differences were observed among mean concentrations of dissolved metals in the individual rivers (F = 0.10-0.84). Contributions from anthropogenic sources are suggested by comparison of trace metal ratios in the samples to crustal abundances. These results establish a baseline for assessing future responses of Eurasian arctic river systems to climate-related environmental changes and shifting patterns of pollutant discharge.